It is known to use an RF confined ion trap upstream of an ion mobility separator (IMS) device in order to increase the duty cycle of the instrument. In particular, ions may be accumulated in the ion trap from an upstream ion source and then pulsed into the IMS device. Whilst the ions are separating within the IMS device it is undesirable to permit further ions to enter the IMS device. During this period, ions from the upstream ion source are accumulated in the ion trap, such that they are not lost and so that the duty cycle of the instrument is improved. These ions may subsequently be pulsed into the IMS device. Ions may therefore be accumulated in the ion trap and periodically released into the downstream ion mobility separation region at the start of each IMS separation cycle.
The ion trap may be operated at a relatively high elevated pressure (e.g., 0.2-20 mbar) that is similar to the pressure used in the IMS device. At such elevated pressures the local charge density within the ion trap increases at the position where the beam of ions enters the ion trap. If the local charge density in the ion trap becomes too high then ions may dissociate due to heating from proximity to the radial confining RF fields. This is a particular problem for thermally labile compounds.
Furthermore, when ions are released from the ion trap into the IMS device, the high charge density caused by the above can cause RF heating in the IMS device and/or distortions in IMS peak width and drift time during separation.
It is therefore desired to provide an improved mass or ion mobility spectrometer, an improved ion trapping system, an improved method of mass or ion mobility spectrometry, and an improved method of trapping ions.